Printed circuit board designers must communicate the manufacturing intentions of a project so that there aren’t any problems in building the board. To do that, consider all ways to ensure that data in PCB design documentation is as straightforward as possible.
| List of PCB Design Documentation | |
|---|---|
| Bill of Materials (BOM) | Includes a component list and all relevant associated information, such as part number, quantity, reference designator, value, etc. |
| Schematic | A logical representation of the circuit layout that connects all components according to the desired function. |
| Board file | The physical realization of the schematic, which graphically places components and routes connections. |
| Mechanical files | Designers may also wish to produce enclosure files for separate production, box builds, etc. |
Clear PCB Design Documentation Starts with Manufacturing Drawings
Most printed circuit board designers have created drawings to accompany their designs–after all, it is part of the job. The problem is that, sometimes, PCB designers don’t know everything that a contract manufacturer needs in those drawings to build the board successfully. Creating a simple drawing using PCB design CAD tools is pretty straightforward, but a simple drawing won’t be enough without the necessary manufacturing data. You can expect your CM to spend much time with you on the phone to fill in the missing details.
Fabrication drawing
The fabrication drawing displays all the drilled holes in the board and is usually auto-generated by the CAD system. Make sure to dimension features such as the board outline and mounting holes, and include specific notes to instruct the fabrication house on the details of the board’s construction. The following should also be present:
- A complete stackup view specifying layer dimensions and board materials. Some detailed board materials are often included in the drawing notes, too.
- Complete drill location information with different drill symbols for each size. A separate drill chart should associate the finished hole size with each symbol.
- Non-standard board features, such as slots or cut-outs, must be fully dimensioned. These often aren’t included in the NC drill file and should be included in the drawing.
- Processing requirements, such as solder mask color or controlled impedance trace routing, should be included in the drawing notes.
Assembly drawing
Like the fabrication, the assembly drawing is also typically auto-generated from the PCB design CAD tools. This drawing shows all of the components on the board in their assembled locations, along with their corresponding reference designators for identification. You will also want to include some notes to instruct the manufacturer on how the board’s assembly will proceed. Depending on the board’s complexity, much more documentation may be necessary. Consider the following when you are creating your next assembly drawing:
- Critical components (e.g., connectors and heat sinks that need specialized mounting instructions) should be called out in the assembly notes on the drawing.
- The drawing notes should include instructions for mounting additional hardware onto the board, such as stiffener bars, handles, or ejectors
- Critical assembly areas should use an exploded view or “detail” that is enlarged and pulled away from the rest of the board image.
- The drawing should also show the location of assembly stickers and labels.
Potential Problem Areas in the Bill of Materials File
Like the manufacturing drawings, bill of materials files are usually auto-generated from the PCB design CAD tools. As such, they should be complete. However, CAD tools give you much flexibility, and leaving important information out of the final bill of materials (BOM) report can be easy. The BOM generator will also precisely report the contents of the design database. If there are part errors, users can locate and report them in the BOM. Here are some examples of items designers can double-check to ensure a BOM file is complete and correct:
- Description mismatch – When the part description in the BOM doesn’t match the description of the part numbers, the PCB assembly slows until the BOM descriptors are verified.
- Missing reference designators – Without reference designators on listed parts in the BOM, the manufacturer doesn’t know which parts get installed and which footprint on the board.
- Incorrect or partial manufacturing part numbers (MPNs) – Trailing dash numbers are sometimes omitted on the BOM, or the part number needs to be corrected, forcing the PCB manufacturer to delay the build until confirmation receipt.
- Incomplete information – In some cases, designers may accidentally omit components from the BOM. This can quickly happen if the PCB designer forgets to update the BOM after a design change. These missing components may only show up in the assembly process once it is clear that kitting is incomplete.
Three Commonly Forgotten Files Your Manufacturer Needs
By going through your documentation and ensuring it is complete with all of the necessary data, you can help your circuit board go through the manufacturing process without any problems. There are three additional files, though, that are important to mention because these three often get left out of the documentation package and can cause problems for the contract manufacturer:
- IPC netlist – This data file is often not sent to the CM because it isn’t a typical Gerber, drill, or BOM file. However, the CM will use this netlist to develop the test fixtures and procedures and identify board fabrication details that need special attention.
- Schematic database – Like the IPC netlist, this database file may not be part of the manufacturing deliverables. However, the schematic database is essential to extract the netlist from layouts, which must be compared to the schematic to resolve design issues.
- Critical components – Include a list in your documentation package of those that require unique fabrication, assembly, or test requirements. Special-order and obsolete components would also fall under this purview. This list lets the CM know which components need attention immediately instead of finding out later during the pre-manufacturing review.
PCB design documentation is critical for the manufacturer since it conveys the project scope and how to arrive there (suggested materials, processes, etc.). Even adept designers may struggle to produce manufacturing-ready documentation that keeps production schedules on target and under budget. Fortunately, VSE has ample experience working with designers; our engineers are committed to building electronics for our customers and conducting a thorough design review as part of manufacturing preparations. We’ve been realizing life-saving and life-changing devices for over forty years with our valued manufacturing partners.
